1. Field of the Invention
The present invention relates to an exposure apparatus for use in a photolithographic process for fabricating semiconductor devices, liquid crystal displays or others.
2. Description of the Related Art
Among various types of exposure apparatuses used for fabricating semiconductor devices and the like, exposure apparatuses, in which a pattern on a reticle (or mask) is transferred onto each of shot areas on a photoresist-coated wafer (or glass plate, etc.) by exposure, are generally provided with an illuminance control mechanism for ensuring that the exposure on each shot area will fall into an acceptable exposure range. The illuminance control mechanism used in such an exposure apparatus is primarily composed of two parts. One is an illuminance distribution control mechanism for suppressing unevenness in the illuminance distribution in an illumination area on the reticle. The other is an exposure control mechanism for maintaining the accumulated exposure in each shot area on the wafer at an appropriate exposure level.
In this regard, one of the most commonly used types of exposure apparatuses is the one-shot exposure type of projection exposure apparatus using the step-and-repeat technique (such as a stepper). In the one-shot exposure type of exposure apparatus, shot areas on the wafer are sequentially positioned at the exposure location, and the pattern on the reticle is transferred through a projection optical system onto the shot area positioned at the exposure location, by performing the one-shot exposure operation, when both the reticle and the wafer is held stationary. For the one-shot exposure type of projection exposure apparatus, the illuminance distribution control may be performed by intermingling a large number of light beams from the corresponding number of light source images by using an optical integrator (such as a fly-eye lens) disposed in the illumination optical system. Further, in a typical one-shot exposure type of projection exposure apparatus, since the exposure is performed on each shot area when the wafer is held stationary, the accumulated exposure in each shot area may be determined by a procedure comprising the steps of: branching out a part of the exposure illumination beam to obtain a monitor beam; continuously receiving the monitor beam during the actual exposure time so as to generate an electrical signal converted from the received beam; integrating the electrical signal to derive an integrated value; and scaling the integrated value by an experimentally determined scale factor.
Thus, the exposure control mechanism for the one-shot exposure type of projection exposure apparatus may be easily constructed. As an example, it may comprise: a photodetector (integrator sensor) for receiving the monitor beam; integrator means for integrating the detection signal produced from the integrator sensor; and control means for controlling either the intensity of the illumination beam or the exposure time such that the difference between the integrated value obtained from the integrator means and the desired value may be reduced.
In this relation, note that there have been proposed and used various sophisticated illumination methods in order to improve resolution and focal depth of the projection exposure apparatus, in particular for a very fine periodical pattern. Those methods include "modified light source method" which uses an illumination-system-aperture-stop comprising a number of openings arranged eccentrically to the optical axis of the illumination system (such as disclosed in U.S. patent application Ser. No. 791,138 filed on Nov. 11, 1991) and "annular illumination method" which uses an illumination-system-aperture-stop having an annular shape. Even when an illumination-system-aperture-stop is replaced with another having a different shape, the actual illuminance on the surface of the wafer may be monitored with precision if the photosensitive surface of the integrator sensor is positioned in a detection plane which is substantially conjugate to the surface of the wafer. Therefore, by controlling an appropriate parameter, such as the exposure time, in such a manner that the integrated value obtained through the integration of the detection signal from the integrator sensor may be converged to a predetermined desired value, the accumulated exposure on each shot area on the wafer can be made to fall into an acceptable range with ease.
More recently, as larger and larger patterns are used for the chips of semiconductor devices, it has become desirable to develop a projection exposure apparatus which is capable of projecting a much larger pattern onto a wafer with a higher efficiency. In order to satisfy this desire, one requirement is to ensure that the distortion will be kept below an acceptable limit over the entire region of an exposure area. Thus, engineer's attention has been drawn to the so-called step-and-scan type of projection exposure apparatus because it is highly effective for suppressing distortion over the entire region of a large exposure area. In the step-and-scan type of projection exposure apparatus, the wafer is stepped to sequentially position the shot areas on the wafer to the scanning start location, and the reticle and the wafer are moved in synchronism with each other and relative to the projection optical system for scanning, so as to serially transfer the pattern on the reticle onto each shot area on the wafer. The step-and-scan type of projection exposure apparatus is a development of the so-called slit-scan type of projection exposure apparatus (such as an aligner), in which a conventional 1:1 projection optical system is used and the reticle and the wafer are moved in synchronism with each other for scanning so as to serially transfer the pattern on the reticle onto the entire region on the surface of the wafer.
A typical illuminance distribution control mechanism (forming a part of illuminance control mechanism) for use in the slit-scan type or the step-and-scan type of projection exposure apparatus may comprise an optical integrator as with the one-shot exposure type of projection exposure apparatus. Where such an optical integrator comprises fly-eye lenses, the entrance surface of each lens element of the final-stage fly-eye lens is positioned in a plane optically conjugate to the plane of the pattern bearing surface of the reticle. Also, the illumination area on the reticle for the scanning exposure has an elongate rectangular shape or an elongate arc shape (this illumination area is referred to as the "slit-like illumination area" hereinafter), so that it is preferable for obtaining a higher illumination efficiency that each lens element of the final-stage fly-eye lens has a cross section of an elongate rectangular shape similar to that of the slit-like illumination area.
On the other hand, a typical exposure control mechanism for use in the scanning exposure type of projection exposure apparatus differs from those for use in the one-shot exposure type of projection exposure apparatus. In the scanning exposure type of projection exposure apparatus, each shot area on the wafer is scanned by the slit-like exposure field having its width (size in the scanning direction) smaller than the length (size in the scanning direction) of the shot area. Therefore, the control of the accumulated exposure in each shot area has to be performed such that the accumulated exposure in the slit-like exposure field may be kept at a constant level at all points on the wafer. If accumulated exposures at different points on the wafer have different values, it means that there is unevenness in the accumulated exposure in each shot area, which provides the same error as the unevenness in the illuminance within the illumination area experienced in the one-shot exposure type of projection exposure apparatus.
For the one-shot exposure type of projection exposure apparatus, one method of controlling the accumulated exposure is to operate the shutter to control the exposure time. For the scanning exposure type of projection exposure apparatus, however, the accumulated exposure at a particular point on the wafer can not be controlled by the operation of the shutter because each shot area is serially exposed by scanning. Therefore, for the scanning exposure technique, the accumulated exposure is controlled by, for example, scanning the reticle and the wafer at controlled, constant velocities. However, any method based on the control of the scanning velocities is not capable of fine adjustment of the accumulated exposure by means of time adjustment. Thus, for the scanning exposure technique, it is required to perform continuous illuminance control during the exposure operation on each shot area, so as to obtain illuminance stability with time. In relation to the illuminance control for the one-shot exposure technique, as one of the known methods for keeping illuminance at a constant level, continuous monitoring of the intensity of the illumination beam and feedback the monitored results to the power supply for the exposure light source is often performed so as to control the input power supplied to the exposure light source from the power supply. This technique is called the "constant-illuminance-control" technique.
Further, there have been proposed other methods of adjusting the quantity of light of the illumination beam by using neutral density filters, or using a mechanism including a pair of shading gratings disposed one on the other wherein the overlaps between the lines of these gratings are adjustable (U.S. Pat. No. 5,191,374).
Moreover, in recent years, there has been developed a technique for enhancing focal depth for a given pattern by reducing the numerical aperture (N.A.) of the illumination optical system and hence the ratio of the numerical aperture of the illumination optical system to that of the projection optical system (i.e., the coherence factor (.sigma.-value)).
When constant-illuminance-control technique mentioned above is used to perform the control for keeping the illuminance at a constant level, the following relation must be fulfilled in order to obtain an acceptable exposure: EQU P=E(L/V), or PV=EL
where:
E stands for the illuminance on the wafer established by the illumination beam (or the energy incident on unit area on the wafer per unit time); PA0 V stands for the scanning velocity of the wafer; PA0 L stands for the width of the slit-like exposure field on the wafer measured in the scanning direction; and PA0 P stands for the sensitivity (or the exposure energy per unit area) of the photoresist on the wafer. PA0 e.sub.0 stands for the desired value of the illuminance.
It is assumed that the width of the slit-like exposure field L is fixed and the sensitivity P of the photoresist may be of any given value. In such a case, acceptable exposures for, photoresists of different sensitivities may be achieved if either the scanning velocity V of the wafer or the illuminance E of the illumination beam is adjustable.
Therefore, as one method, the control of the accumulated exposure may be performed by adjusting the illuminance E of the exposure illumination beam. In such a case, it is required to measure the illuminance on the wafer with a short measuring time and precisely by means of an illuminance unevenness sensor (such as a photodetector), and control the output power of the light source for the illumination beam, or the attenuation of the illumination beam established by the attenuation unit disposed on the path of the illumination beam path, or other appropriate factors.
FIGS. 28(a) and 28(b) show photosensitive areas of two different conventional illuminance unevenness sensors provided on the wafer stages. FIG. 28(a) shows an example in which an illuminance unevenness sensor has a slit-like photosensitive area. In FIG. 28(a), there is shown an exposure field 402 having an elongate rectangular shape, extending in the non-scanning direction (the Y-direction) and substantially inscribed in a circular shaped effective exposure field 401 of a projection optical system. The slit-like photosensitive area 403 of the illuminance unevenness sensor has an elongate rectangular shape and extends in the scanning direction. The photosensitive area 403 is moved in the non-scanning direction along a solid line arrow in the figure to scan the exposure field 402, so as to detect any illuminance unevenness in the non-scanning direction. The length DA of the photosensitive area 403 is greater than the width LA of the exposure field 402 in order to complete the exposure measurement in the exposure field 402 with a single pass scan. FIG. 28(b) shows an example in which an illuminance unevenness sensor has a pinhole-like photosensitive area. In FIG. 28(b), the pinhole-like photosensitive area 404 is moved two-dimensionally as shown by the scan lines 405 in the figure to scan the exposure field 402, so as to measure the exposure distribution over the entire exposure field 402.
Unfortunately, it is not always possible to cause the accumulated exposure at each point on the wafer to fall into an acceptable range in a scanning exposure type of projection exposure apparatus, by using a constant-illuminance-control technique commonly used in the one-shot exposure type of projection exposure apparatus. More particularly, exposure light sources include electric-discharge lamps (such as a mercury-vapor lamp) and laser sources (such as excimer lasers and harmonic generators of YAG lasers). All of these types of light sources require considerably high input power, resulting in a drawback that it is difficult to adjust the illuminance of the illumination beam with high resolution and with high response speed by controlling only the input power supplied to the exposure light source.
This may not be a problem in the one-shot exposure type of exposure apparatus, in which even when the illuminance of the illumination beam from the exposure light source produces a stepwise variation, the accumulated exposure can be made to fall into an acceptable range with ease by merely controlling the shutter to adjust the exposure time in response to the variation. However, in the scanning exposure type of exposure apparatus, it is difficult to adjust the accumulated exposure by controlling the shutter because the exposure operation is continuously performed.
In view of the foregoing, it is a first object of the present invention to provide an exposure apparatus of the scanning exposure type in which even when the setting resolution in the input power supplied to an exposure light source is relatively low, the accumulated exposure on a photosensitized substrate can be made to fall into an acceptable range.
It is a second object of the present invention to provide an exposure apparatus of the scanning exposure type in which even when the setting resolution in the input power supplied to an exposure light source is relatively low, the illuminance of the illumination beam may follow the desired value with high response speed and with precision, with the result that the accumulated exposure on a photosensitized substrate can be made to fall into an acceptable range with ease.
With regard to a third object of the present invention, in the case where the exposure light source comprises an electric-discharge lamp utilizing an arc-discharge, such as a mercury-vapor lamp, there may occur variations in the illuminance of the illumination beam due to power supply noises, as well as there may occur such variations in the illuminance of the illumination beam known as "arc fluctuations" depending on the conditions of the convection of the gas filled in the tube of the electric-discharge lamp. These illuminance variations (fluctuations) are relatively small in amplitude. However, if such variations include high frequency components, a constant-illuminance-control technique for controlling the input power to the electric-discharge lamp may not follow the variation.
Therefore, when illuminance variations having high frequencies occur due to power supply noise during the scanning exposure operation of one shot area, the accumulated exposures at some points on that shot area may not fall into an acceptable range. This constitutes one of the factors in a reduced yield of the final products of semiconductor devices and the like. Even when a laser source such as an excimer laser (krypton-fluorine excimer laser, argon-fluorine excimer laser, etc.) or a harmonic generator of YAG laser is used as the exposure light source, the output of the light source may produce variations having high frequencies for some reason, so that the accumulated exposures at some points in each shot area may not fall into an acceptable range. Therefore, even when a laser source is used as the exposure light source, some kind of mechanism for suppressing illuminance fluctuations is required.
In the case where the quantity of light of the illumination beam is adjusted using neutral density filters, the adjustment can be made only in a stepwise manner by changing the filters. Further, the technique using a pair of shading gratings described above has a drawback that the dynamic range for continuously adjusting the quantity of light is narrow.
Further, in the case where the quantity of light of the illumination beam is adjusted by controlling the numerical aperture (N.A.), there has been a drawback that the illuminances on the reticle and on the wafer established by the illumination beam are lowered when the numerical aperture is reduced.
In view of the foregoing, it is a third object of the present invention to provide an exposure apparatus of the scanning exposure type in which even when an illumination beam outputted from an exposure light source produces illuminance variations which are smaller than a predetermined proportion of the normal illuminance and of high frequencies, the accumulated exposure on a photosensitized substrate can be made to fall into an acceptable range with ease.
In relation to the third object above, the present invention provides an exposure apparatus which is capable of highly suppressing the illuminance unevenness of the illumination beam (i.e., the unevenness in the accumulated exposure in each shot area), as well as capable of continuously adjusting accumulated exposure on the photosensitized substrate with a wide dynamic range for adjustment.
Also, in relation to the third object above, the present invention provides an exposure apparatus which is capable of preventing any reduction in the illuminances on the reticle and on the wafer established by the illumination beam, even when the numerical aperture of the illumination optical system is reduced, i.e., the ratio of the numerical aperture of the illumination optical system to that of the projection optical system, or the coherence factor (.sigma.-value), is reduced.
With regard to a fourth object of the present invention, arc fluctuations described above may appear as illumination variations at frequencies of about 30 Hz, for example. When arc fluctuations start appearing, the previous relationship between the input power supplied to the electric-discharge lamp and the illuminance of the illumination beam is no longer maintained, so that an ordinary constant-illuminance-control operation, in which the input power supplied to the electric-discharge lamp is controlled based on the actually measured illuminance, can not ensure that the accumulated exposure on a wafer will fall into an acceptable range.
Therefore, if arc fluctuations start appearing during the scanning exposure operation of a shot area, the accumulated exposures at some points in that shot area may no longer fall into an acceptable range. This constitutes another of the factors in a reduced yield of the final products of semiconductor devices and the like.
In view of the foregoing, it is a fourth object of the present invention to provide an exposure apparatus of the scanning exposure type in which even when the illumination beam outputted from the exposure light source produces fluctuations in its illuminance, the accumulated exposure on a photosensitized substrate can be made to fall into an acceptable range.
Further, where a sensor having an elongate, slit-like photosensitive area 403 which is longer than the width of the exposure field 402 in the scanning direction (FIG. 28(a)) is used for measuring the illuminance within the exposure field, a high processing speed may be obtained but a large space for the sensor is required on the wafer stage, which is an inconvenient drawback. In addition, the use of such elongate, slit-like photosensitive area has another drawback that the measurement may be made only with low accuracy because the long photosensitive area tends to result in high unevenness in the sensitivity of the sensor. On the other hand, where a sensor having a pinhole-like photosensitive area 404 (FIG. 28(b)) is used for this purpose, only a small space for the sensor is required on the wafer stage but a low processing speed results because the number of the measurement points must be large.
In view of the foregoing, it is a fifth object of the present invention to provide an exposure apparatus of the scanning exposure type, having an illuminance unevenness measurement means which provides rapid measurement and requires only a small space.